Accumulation and Release of Trace Inorganic Contaminants from Biofilm Matrices Produced and Challenged Under Drinking Water Distribution System Conditions

Over the legacy time frames experienced by drinking water distribution systems(DWDS), trace inorganic contaminants (TICs) such as arsenic, lead, antimony, thallium,and chromium can accumulate in high concentrations within a biofilm matrix containing iron (Fe) and manganese (Mn) oxyhydroxides. Release of TICs can occur when thesematrices are disturbed by a physical or chemical change. A bench scale factorial designusing recirculating glass columns (25 mm x 300 mm) with abraded 3 mm glass beads, wasused to establish biofilm matrices under a standard conditions at 16°C, 0.2 mg/l chlorine(Cl2,) and 1 mg/L dissolved organic carbon (DOC) for 6 months. The developed matrixwas then challenged by altering the temperature (7 and 25°C) and concentrations of Cl2 (0and 2 mg/L) and DOC (0 and 2 mg/L). During this 4 week challenge phase, effluent TICconcentrations were monitored, after which each column was sacrificed and analyzed forATP and DNA, and TIC distribution within the solids using sequential extractions and totaldigestions. Solid phase Mn and Fe were associated with the organic fraction of the biofilmmatrix. With increased Cl2, more accumulation of Mn occurred due to the oxidation of Mn(II) to Mn(IV). This resulted in increased surfaces for sorption of Fe and the TICs, Pb,Tl, Cu, and Cr. Increased Cl2 also resulted in a less active biofilm as indicated by lowerATP concentrations within the biofilm matrix. Less As accumulated with increased DOCas a result of surface site competition. Lead was impacted by Cl2 and DOC, where theaddition of Cl2 increased the retention of lead while the addition of DOC caused a decrease.The outcome of this investigation was the identification of conditions that causeaccumulation of TICs to biofilm matrices under DWDS conditions, demonstrating a highpotential for accumulation when the DWDS is sourced with Fe/Mn and TICs at secondaryand primary MCLs. Though not the focus of this study, these highly concentrated solids are susceptible to physical disturbance and release showing the importance of both minimizing accumulation and conducting regular pipe cleaning activities